1. Field of the Invention
The present invention relates to a method for measuring and evaluating a variety of inherent measurement errors of a coordinate measuring machine which is used to measure, for example, dimensions of mechanical parts, etc., and to a gauge for a coordinate measuring machine, which is adapted to measure an error of a coordinate measuring machine.
2. Description of the Related Art
In a known coordinate measuring machine having a movable probe capable of moving in three mutually orthogonal directions, the tip of the movable probe is brought into contact with an object to be measured that is set on a measuring table in order to measure dimensions of the object. The object may be, for example, a mechanical part such as a casing for an engine or for a transmission gear box.
In general, in such a coordinate measuring machine, the probe is movable in three mutually orthogonal directions. For example, Japanese Unexamined Patent Publication No. H02-306101 discloses a coordinate measuring machine in which a first gantry type movable member is linearly movable along horizontal guide rails which extend on opposite sides of a measuring table on which an object to be measure is set in place. The first movable member is provided with a second movable member mounted thereon so as to move in a horizontal direction perpendicular to the direction of the movement of the first movable member.
The second movable member is provided with a vertically movable spindle portion whose front end has a probe with a ball secured thereto. The probe is moved in the three-dimensional directions while bringing the ball into contact with the upper surface of the object to be measured, which is set on the measuring table, to measure the dimensions of each part of the object.
In the coordinate measuring machine as discussed above, if the ball of the probe is worn, correct measurement can no longer be expected. To prevent this, a reference gauge is set on the measuring table at the interval of the measurement, so that the dimension of each part of the reference gauge is measured to correct an error due to the wearing of the ball of the probe.
The measurement errors of the coordinate measuring machine include those caused by a winding movement of the probe tip which is in turn caused by a deflection or distortion of a guide member, such as the guide rails along which the probe tip is moved or angular deviations from a right angle of two guide members for guiding the movement of the probe in two mutually orthogonal directions, etc.
In the prior art, the straightness of the guide members of the coordinate measuring machine or the orthogonality of the guide members is established by reference gauges which are set in different directions on the measuring table. Therefore, the measuring operation for evaluation of the errors of the coordinate measuring machine requires time and labor.
In recent years, on the one hand, the operating efficiency of the coordinate measuring machine has been increased in companies or factories to determine the dimensions of precisely and complicatedly machined workpieces, and on the other hand, the coordinate measuring machines tend to be continuously used without periodically checking the performance thereof, from the economic viewpoint or in the view of practical use.
It is an object of the present invention to eliminate the above-mentioned drawbacks of the prior art by providing a method for evaluating a measurement error in which an error evaluation in connection with the straightness of the machine axes in a coordinate measuring machine or the orthogonality of the machine axes can be easily and precisely carried out.
Another object of the present invention is to provide a gauge for a coordinate measuring machine, for use with the measurement error evaluating method.
A measurement error evaluating method for a coordinate measuring machine according to the present invention is applied to an error evaluation of a coordinate measuring machine in which a probe tip is moved relative to an object to be measured along three different mutually orthogonal axes.
According to one aspect of the present invention, there is provided a method for evaluating a measurement error of a coordinate measuring machine, in which a tip of a probe is moved relative to an object to be measured, along three orthogonal machine axes, comprising;
a first step of setting on a measuring table of a coordinate measuring machine a gauge for the coordinate measuring machine having a plurality of balls whose centers are aligned on a straight line inclined with respect to a reference axis set in a virtual reference plane and extending in the virtual reference plane so that the reference axis is in parallel with one of the three machine axes of the coordinate measuring machine and so that the virtual reference plane is in parallel with either one of the remaining two machine axes of the coordinate measuring machine;
a second step of setting orthogonal coordinates in which one of the coordinate axes is identical to the direction of the reference axis in the virtual reference plane, so that the center position of each ball with respect to the coordinates is measured by the coordinate measuring machine;
a third step of turning and inverting the gauge for the coordinate measuring machine about the reference axis by 180 degrees and again setting the gauge on the measuring table of the coordinate measuring machine; and
a fourth step of setting orthogonal coordinates in which one of the coordinate axes is identical to the direction of the reference axis in the virtual reference plane, so that the center position of each ball with respect to the coordinates is measured by the coordinate measuring machine.
The straightness of the machine axis in the reference axis direction can be evaluated, based on coordinates Yi of the center of the i-th ball in a direction perpendicular to the reference axis of the gauge for a coordinate measuring machine, obtained in the second step, and coordinates Yxe2x80x2i of the center of the i-th ball in a direction perpendicular to the reference axis of the gauge for a coordinate measuring machine, obtained in the fourth step, by calculating the difference between the maximum value and the minimum value of (Yixe2x88x92Yxe2x80x2i)/2.
Also, in an embodiment, a regression line is obtained from the coordinates of the centers of the balls in the reference axis direction of the gauge and the direction perpendicular to the reference axis, obtained in the second step, to thereby calculate an angle xcex8 between the reference axis and the regression line, and thereafter a regression line is obtained from the coordinates of the centers of the balls in the reference axis direction of the gauge and the direction perpendicular to the reference axis, obtained in the fourth step, to thereby calculate an angle xcex8xe2x80x2 between the reference axis and the regression line, so that the orthogonality of the two machine axes parallel with the virtual reference plane is evaluated using (xcex8xe2x88x92xcex8xe2x80x2)/2.
According to another aspect of the invention, there is provided a gauge for a coordinate measuring machine comprising;
a plurality of balls with which a tip of a probe of a coordinate measuring machine is brought into contact, and
a holder which holds the balls arranged along a line inclined with respect to a reference axis set in an virtual reference plane and extending in the virtual reference plane, said holder being capable of attachment to the coordinate measuring machine so that the virtual reference plane is parallel with two optional machine axes of the coordinate measuring machine and that the reference axis is parallel with one of the two machine axes.
Preferably, the holder is made of a trapezoidal block, so that the balls are arrayed and mounted along each oblique line parallel with the non-parallel oblique surfaces of the trapezoidal block.
Preferably, the holder is made of a block having a trapezoidal through hole, so that the balls are arrayed and mounted along each line parallel with opposite non-parallel oblique surfaces of the through hole.